Gastric cancer or stomach cancer refers to tumors that develop in the lower part of the esophagus, in the stomach or in the uppermost part of the small intestine. Gastric cancer is a leading cause of cancer-related death worldwide in which almost one million new cases are being diagnosed yearly (Lam, K. W., Lo, S. C., Proteomics Clin. Appl. 2008, 2, 219-228). The global 5-year survival rate is around 20% except for Japan with close to 60% (Kamangar F, et al. J Clin Oncol 2006; 24:2137-50). Interesting discrepancies between the Western and Eastern populations had been reported in terms of the frequency of early detection and prognosis of the disease, probably resulting from the differences in gastric cancer epidemiology, staging systems and treatments (Davis, P. A., Japanese journal of clinical oncology 2000, 30, 463-464). Early detection is believed to be a key pillar in the management of gastric cancer. Population screening by endoscopy introduced in Japan and other Eastern countries leads to almost 70% of gastric cancer being diagnosed in early stage compared to merely 15% in Western countries where screening remains elusive (Cunningham and Chua, The New England journal of medicine 2007, 357, 1863-1865). However the cost effectiveness of population screening in other countries apart from Japan and Korea remains questionable. Screening of high risk subjects may be a viable alternative (Yeoh, K. G., Journal of gastroenterology and hepatology 2007, 22, 970-972. Leung, W. K., et al., The lancet oncology 2008, 9, 279-287). However, there is currently no specific biomarker available for gastric cancer screening and diagnosis in the clinic and the commonly used markers such as CA19-9, fetoprotein antigen, pepsinogen I/II, carcinoembryonic antigen (CEA) etc are insensitive (Lam and Lo 2008). Clearly there is a need to find better molecular markers.
Conventional tumor markers such as CA19-9, CA72-4 and CEA are not adequately sensitive and/or specific for gastric cancer detection. A recent review summarized the tumor marker sensitivity in gastric cancer detection including CEA at 16% to 63%, CA19-9 at 20% to 56% and CA72-4 at 18% to 51% (Ebert, M. P., Rocken, C., European journal of gastroenterology & hepatology 2006, 18, 847-853). The specificity of these markers was not defined. M2-pyruvate kinase (M2-PK), described as tumor-associated metabolic marker, had also been evaluated for gastric cancer detection, with the sensitivity and specificity ranged from 57% to 67% and 89% to 95% respectively (Kumar, Y., et al. European journal of gastroenterology & hepatology 2007, 19, 265-276.;] Hardt, P. D., et al. Anticancer research 2000, 20, 4965-4968.; Cerwenka, H., et al., Anticancer research 1999, 19, 849-851. In general, 2 conclusions could be derived from these reports: i) the current tumor markers candidates have a sensitivity of less than 67% for gastric cancer and ii) most of them are not specific for any cancer type.
Classification of gastric cancer is often based on the Lauren classification. This is probably the most successful and widely used today (Vauhkonen, M., et al. Best practice & research 2006, 20, 651-674). Based on Lauren classification, gastric cancer can be classified into two main cancer pathogeneses:—(i) intestinal (IGCA) and (ii) diffuse (DGCA) subtypes. These two subtypes show significant differences in epidemiologic and prognostic features, which excite clinicians and oncologists to pursue further understanding of the basis for classification (Kountouras, J., et al. Hepato-gastroenterology 2005, 52, 1305-1312). The proportion of intestinal type (IGCA) accounts for approximately 50%, that of the diffuse type (DGCA) 35% and the remainder 15% is characterized as “unclassified” or mixed type cancer. The intestinal type (IGCA) is characterized by cohesive neoplastic cells forming gland like tubular structures, whereas in diffuse type (DGCA) cell cohesion is absent, so that individual cells infiltrate and thicken the stomach wall without forming a discrete mass. This difference in microscopic growth pattern is also reflected in the different macroscopic appearance of the two histological subtypes. Whereas for intestinal type (IGCA) the macroscopic margins correspond approximately to the microscopic spread, the diffuse type (DGCA) as a poorly differentiated cancer can extend submucosally far beyond its macroscopic borders. This difference in tumor spread of the two types of Lauren-classification is of clinical importance in decision-making about appropriate treatment options. The intestinal type (IGCA) predominate in high-risk areas, occur more often in distal stomach, and is often preceded by a prolonged precancerous phase, whereas diffuse type (DGCA) tumors prevail among young patients and women and the contribution of hereditary factors to their causation is higher. Classification of gastric cancer based on the Lauren classification requires invasive sampling methods.
The advancement in analytical tools and mass spectrometry platforms has spurred the quest of biomarker discovery. Proteomics approaches in unearthing biomarkers have shown successes in breast, prostate, lung, ovarian cancer and to a smaller extent in gastric cancer in which potential candidates have been identified from tumour tissue (He, Q. Y., et al., Proteomics 2004, 4, 3276-3287) and cell lines (Takikawa, M., et al., Oncology reports 2006, 16, 705-711). For example, one study employed two-dimensional gel electrophoresis (2-DE) approach to profile disease-specific protein expression from gastric juice (Lee, K., et al., Proteomics 2004, 4, 3343-3352). Another 2-DE approach further identified 14 differentially expressed proteins in gastric cancer versus normal tissues (Ryu, J. W., et al., Journal of Korean medical science 2003, 18, 505-509). Although studies on tumour and gastric juice had provided great insights on the disease, biomarker discovery based on these approaches engage invasive sampling methods and are not ideal from a clinical point of view.
Some groups have mined blood samples for biomarkers using a ProteinChip system, which is based on the surface enhanced laser desorption/ionization (SELDI) approach (Liang, Y., et al., Experimental and molecular pathology 2006, 81, 176-180); (Ebert, M. P., et al., Journal of proteome research 2004, 3, 1261-1266); (Poon, T. C., et al., Gastroenterology 2006, 130, 1858-1864). Although differences were found in the peptide mass fingerprint, this information is incomplete without knowing the identities of the protein. Although, a recent paper had successfully developed the methodology to identify SELDI profile peaks using ProteinChip coupled with a tandem mass spectrometer (Peng, J., et al. Proteomics 2009, 9, 492-498) one should be cautioned that reproducing the serum profiling using SELDI could be difficult due to various intrinsic or extrinsic factors. On the other hand, one study employed conventional 2-DE gel coupled with mass spectrometric analysis and revealed that the up-regulation of cathepsin B in the sera of gastric cancer patients can be used as a prognosis marker but not for early diagnostic (Ebert, M. P., et al., Proteomics 2005, 5, 1693-1704). However, sensitivity remains an issue with 2-DE approach and detection of low abundance proteins remains challenging. This problem is further amplified by the fact that proteins in the blood have a wide protein dynamic range spanning over 10 orders of magnitude.
The typical role of C9 is in the innate immune system, which is one of the host's defense systems against foreign bodies. C9 is a part of the terminal pathway in the complement system and together with C5b, C6, C7 and C8 is required for the assembly of membrane attack complex leading to cell lysis.